In addition to providing telephonic services, wireless telecommunications networks provide a number of supplementary services. One such service is a group call, wherein a member of a predefined group can set up a call to each member of the group. For example, in response to an emergency, a law enforcement dispatcher can notify law enforcement personnel of the emergency using a group call, where the law enforcement personnel are members of a common group.
Ordinarily, a group call is set up for each member of the group, regardless of their location. Therefore, a group conference could be established in which members are dispersed over a large area, which can be a disadvantage in the situation where the initiator of the group call needs to reach only the members of the group within a certain area. To limit the group call to a particular area, a list of cells in which members of the group are to be called is maintained. The list of cells are collectively known as the group call area. Members outside the group call area are not called. The list of cells is stored in a database that stores any number of group identification numbers, followed by identifiers of the particular cells in which members of the identified group are to be called.
When a group call is requested to be established by a mobile station belonging to a particular group, the list of cells in the group""s group call area is retrieved. Within each cell of the group call area, a notification message is transmitted to each mobile station in the cell over a control channel. The notification alerts the mobile stations of the group call, identifies the group to receive the call, and identifies the traffic channel for the call. Subscribers at mobile stations belonging to the group are alerted and can then receive the group call by tuning to the indicated traffic channel.
In this methodology, group members outside the group call area are not called during a group call. However, where a large number of group members are only temporarily outside the group call area, it may be advantageous to call the group members upon their return to the group call area. Accordingly, to maximize the number of group members receiving the group call, the voice message is repeated several times on the traffic channel while the notification message is continuously transmitted over the control channel. In this manner, absent group members temporarily outside the group call area during one repetition of the group call are notified upon their return to the group call area. Although continuously transmitting the notification message and repeating the voice message maximizes the number of group members receiving the group call, the foregoing is annoying to group members who have already listened to one repetition of the voice message. Because the notification message is continuously transmitted during the repetition of the voice message, even mobile stations that have heard one repetition of the voice message are alerted by the notification message.
Another technique to maximize the number of group members receiving the group call is to have listening subscribers call an acknowledgment center to acknowledge the group call. For example, according to the Union Internationale des Chemins de Fer (UIC) MORANE specifications, for a GSM-Railway (GSM-R) system, an acknowledgment center in the network would record and process acknowledgments to group calls. Upon finishing the group call, the listening mobile station establishes a point-to-point call with the acknowledgment center to acknowledge listening to the group call. Although the acknowledgment center maintains records of mobile station which acknowledge listening to a group call, there is no mechanism for distinguishing which group call was listened to. The foregoing is especially problematic in situations where a particular group member, for example, a law enforcement dispatcher, makes frequent group calls on an ongoing basis.
Accordingly, it is an object of the present invention to provide a mechanism wherein a mobile station can ascertain whether an incoming group call has already been listened to by the mobile station.
It is also an object of the present invention to provide a mechanism whereby a group call initiator can determine which group members have listened to a particular group call.
The present invention is directed to a system, method, and apparatus for receiving a group call at a mobile station, wherein a notification message including a group identification number and a counter is detected on a control channel. When the notification message is received by a mobile station, the mobile station examines the counter and checks the contents of a group call history memory. Where the contents of the group call history memory include the counter value, the mobile station determines that the notification message is for a group call that was already received by the mobile station and is being retransmitted, and ignores the group call.